During some semiconductor processes, substrates or wafers are processed by deposition. For example, in plasma-enhanced chemical vapor deposition (PECVD) or plasma-enhanced atomic layer deposition (PEALD), a process gas is admitted into a reaction chamber held at a reduced pressure, and oscillatory electric energy at a radio frequency (RF) is applied to the chamber to excite the gas into a plasma. The gas reacts with the surface of a substrate or wafer exposed to the plasma to form a film on the wafer of a composition derived from that of the process gas. The film formed on the wafer or the substrate is desired to have good uniformity. Accordingly, the semiconductor apparatus for the semiconductor process must be designed to ensure good uniformity for the film deposited on the wafer or the substrate.